This invention relates to a developing apparatus adapted for visualizing with a toner an electrostatic latent image formed on a photosensitive drum by the electrophotographic method.
Heretofore, as means for visualizing an electrostatic latent image formed on a photosensitive drum with a toner, a contact type developing apparatus having an essential construction as illustrated in FIG. 1, for example, has been known. With the developing apparatus of this kind, the development is accomplished by the following operation.
A toner 2 placed in a toner container 1 is supplied onto a developing roller 5 by toner supplying means which comprises a stirrer 3, a toner supplying roller 4, etc. The toner 2 supplied to the developing roller 5 is retained in the form of a thin layer on the surface of the developing roller 5 and, at the same time, vested with a fixed magnitude of electric charge by being passed through the area of a toner distribution and charging blade having the terminal part thereof disposed closely to the peripheral surface of the developing roller 5 and having the height thereof adjusted by a spring 6a. An electrostatic latent image formed on a sensitive drum 7 by an exposure device omitted from illustration in the diagram is developed by causing the toner deposited on the surface of the developing roller 5 to adhere to the photosensitive drum 7 by means of static electricity. In this manner, the electrostatic information recorded on the photosensitive drum 7 is visualized. The visible image is recorded on a recording paper by a transfer device 8, for example. The residue of the toner remaining on the developing roller 5 is recovered into the toner container 1 by a recovery blade 9. In the diagram, 10 stands for an electrification device and 11 for a cleaning unit.
The toner distribution and charging blade mentioned above is generally formed of a rubber from among urethane rubber, silicone rubber, chloroprene rubber, butadiene rubber, isoprene rubber, acrylonitrile-butadiene rubber, ethylene-propylene rubber, and natural rubbers, a varying metal from among iron, copper, aluminum, stainless steel, and alloys thereof, and a varying resin from among phenol resin, hard vinyl chloride resin, polycarbonate resin, polyacetal resin, fluorine resin, and nitro-cellulose resin. The recovery blade 9 is produced by molding polyester, Teflon (tetrafluoroethylene resin), hard vinyl chloride, coated glass cloth, or acetate in the form of a tape or sheet and optionally coating the surface of the tape or sheet with a coating material.
The developing roller 5 mentioned above is generally constructed, as illustrated in FIG. 2, by superposing on a metallic roller substrate 5a an elastomer layer 5b of oilproof rubber and superposing on the elastomer layer 5b an electroconductive layer 5c. This electroconductive layer 5c is generally formed by coating the elastomer layer 5b with an electroconductive coating material having an electroconductive filler dispersed in a commercially available solvent-soluble binder such as an electroconductive urethane type coating material or an electroconductive acrylic type coating material. It has a resistivity approximately below 10.sup.10 .OMEGA..multidot.cm.
Incidentally, in the developing apparatus constructed as described above, it has been heretofore held that for the purpose of conferring a suitable electric charge on the toner, the triboelectric charge imparting members, such as the developing roller, the toner distribution and charging blade, the recovery blade, the toner supplying roller, and the toner container against which the toner particles produce friction are desired to be opposite in charging characteristics to the toner. The electrostatic charging property of the electroconductive layer (applied layer of an electroconductive coating material) of the developing roller, for example, is found by measurement in most cases to be equal in polarity to the toner or, if opposite in polarity, to be substantially destitute of a charging ability. The electroconductive layer, therefore, is incapable of conferring a proper triboelectric charge on the toner and is consequently liable to entail the disadvantage of inferior formation of the toner layer resulting in missing portions from a developed image or uneven thickness of the toner layer or inferior conveyance of the toner. Thus, the failure of the electroconductive layer to form a practically satisfactory image has been frequently encountered.
The impartation to the toner distribution and charging blade or the recovery blade of the same degree of triboelectric charge as to the developing roller results in the following disadvantage. First, the toner tends to adhere to the toner distribution and charging blade or the recovery blade and the amount of the toner to be carried or transported by the developing roller decreases. Then, the portion of the toner which escapes being transferred to the photosensitive drum and survives consumption in image development defies recovery from the developing roller and the recovery blade into the toner container, with the result that a surplus of the toner spills down the toner container. The impartation to the toner distribution and charging blade or the recovery blade of the same degree of triboelectric charge as to the toner results frequently in the failure of the toner to produce a practically sufficient image because the triboelectric charge is improper for the toner and liable to impede satisfactory formation of a toner layer or satisfactory transfer of the toner.
The following references have disclosure regarding the material of the toner distribution and charging blade and the relation between the toner distribution and charging blade and the triboelectric charge of the toner.
U.S. Pat. No. 3,731,146 issued May 1, 1973 to A. C. Betting et al. discloses a doctor blade made of a material remote from the toner in the triboelectric series and close to the development roller surface material in the triboelectric series. This reference, however, has no disclosure about the fact that the triboelectric charge of the toner and the triboelectric charge of the developing roller surface are opposite in characteristic.
U.S. Pat. No. 4,336,318 issued Jun. 22, 1982 to H. Hukumoto et al. discloses that the triboelectric charge of the toner q (c/g) and the thickness of the toner particle layer d (mm) are adjusted to satisfy the following formula in a method for developing electrostatic images: 3.times.10-8&lt;.vertline.q.times.d .vertline.&gt;5.times.10.sup.-6.
This references has no disclosure regarding the interrelation among the triboelectric charge of the toner, the triboelectric charge of the toner distribution and charging blade surface, the triboelectric charge of the developing roller surface, and the triboelectric charge of the recovery blade surface.
U.S. Pat. No. 4,883,058 issued May 23, 1989 to Y. Hirano et al. discloses a blade composed of a silicone rubber comprising 100 parts by weight of a siloxane polymer having a cross linking density of 4 to 8.times.10.sup.-4 mol/cc and 30 to 70 parts by weight of silica. This references, similarly to that of U.S. Pat. No. 4,336,318, fails to disclose anything about the interrelation among the triboelectric charge of the toner, the triboelectric charge of the toner distribution and charging blade surface, the triboelectric charge of the developing roller surface, and the triboelectric charge of the recovery plate surface.